A conventional magnetic rotational-angle detector is adapted to detect, with a magnetic sensor, a magnetic field caused by a magnetic medium which is alternately magnetized with a magnetization pitch λ on an outer periphery of a rotating drum, as illustrated in FIG. 1 in a Patent Document 1, for example. The magnetic sensor has magneto-resistive devices (MR device). This magnetic sensor utilizes characteristic of the MR device that an electric resistance of the MR device is changed by the magnetic field. A rate of the change of the resistance in the MR device depends on a magnitude of the magnetic field, substantially regardless of a direction of the magnetic field. The MR device detects change of intensity of the magnetic field associated with the rotation of the rotating drum, which enables detection of the rotational angle of the rotating drum on the basis of a sinusoidal output signal generated by the MR device.
Although enhancement of the resolution and the accuracy in the detection of the rotational angle can be realized by interpolating the sinusoidal output signal electrically from the magnetic sensor, this is enabled on the condition that the sinusoidal output signal outputted from the magnetic sensor contains small distortions and, therefore, are close to an ideal sine wave. However, the output from the individual MR device has a non-linear relation between the intensity of the magnetic field and the sensitivity characteristic of the MR device. Therefore, in many cases, an output waveform from the magnetic sensor is not the ideal sine wave and contains harmonic distortions superimposed thereon.
In order to eliminate the aforementioned harmonic components from the sinusoidal output signal from the magnetic sensor, it is possible to realize this as follows.
That is, with respect to the n-th order harmonic, outputs from the devices spaced apart from each other with λ/n can be subtracted from each other or outputs from the devices spaced apart from each other with λ/(2n) can be summed up in order to cancel the n-th order harmonic component.
As described above, owing to the arrangement of the MR devices and the operation of subtraction or summation of the signals, it is possible to obtain output signal close to the ideal sine wave by eliminating the harmonic components from the output signal of the magnetic sensor, thereby enabling detection of the position and the rotational angle of the rotating member with high accuracy.    Patent Document 1: Japanese Patent No. 2529960